How does Population Doubling Time explain population changes within a space?

Doubling time in reference to population is used to study the population growth. It projects the amount of time that the population will take to double in a particular region and it is calculated keeping in mind the annual growth rate of population of that area. This can be calculated by the rule of 70. But it is very rare that the doubling time remains the same for long and it is also rare that it changes drastically.